Where did all these jellyfish come from?

The Question: While walking along a busy section of Boston Harbor the other day, I noticed hundreds of jellyfish floating on the surface. I walked about a half mile stretch of water, and they were everywhere. They all seemed to be the same kind – translucent white, with four circular bits in the center of their bodies. There were all different sizes, too. It was an extremely sunny and warm day, which I first thought might be the reason, but the next couple have been cool and rainy, and they are still on the surface! Any ideas?

Submitted by: Marion, Boston, Massachusetts, USA

The Short Answer: What you saw are floating medusae of the moon jelly (Aurelia aurita), one of the most common jellyfish in the world. They like protected waterways, and are tolerant of the wide range of salinity and temperature that you often find in harbors and bays. They also become larger and more visible in the summer, so they are a common sight in city harbors around the globe.

Many species of jellyfish, including moon jellies, have increased in the last decade. Factors that probably favor moon jellies and other species include manmade objects in the water, warmer temperatures, overfishing, and fertilizer runoff. Here’s a video from Boston Harbor in summer 2010 that shows the jellyfish:

More Information: Here’s how the four factors mentioned above favor jellyfish:

Manmade objects in seawater – most jellyfish, including moon jellies, have a polyp stage that attaches to a hard object such as a rock. Polyps generally can’t settle on shifting sand, or organic sediment. By building structures into the water, such as piers and docks, and by allowing plastic and other manmade trash to be washed into our harbors and bays, we provide lots of hard surfaces for jellyfish polyps.

Warmer water due to climate change – moon jellies develop faster in warmer water, so increases in temperature due to climate change shorten the time between generations. In addition, the cold water of winter usually kills the swimming moon jellies, leaving only the attached polyps to start the next generation. With warmer temperatures, however, some moon jellies survive winter, giving them a head start to begin reproducing when the water warms up in the spring.

Overfishing – fish larvae and moon jellies compete for copepods, the tiny innumerable crustaceans that feed on plankton. When fish and jellyfish compete, the growth of jellyfish is limited. Take away the fish – as we have in so many ways all around the world – and the jellyfish populations grow rapidly. To make matters worse, jellyfish like moon jellies will actually prey on fish eggs and larvae. When there aren’t that many moon jellies and other jellyfish, the fish larvae can compete for copepods. But there is some evidence that once the number of jellyfish gets above some threshold, they limit the ability of fish to reproduce, thereby slowing the return of a balance between fish and jellyfish.

Fertilizer runoff – When agricultural and lawn and garden fertilizer runs into harbors and bays, it can lead to blooms in algae, dinoflagellates, and other microorganisms. This changes the food web in ways that favor jellyfish over fish.

The Moon Jelly – One of nature’s most adaptable creatures

The list above makes it clear that there are man-made changes going on that favor moon jellies. But give some credit to the jellyfish themselves for being able to exploit those changes. They can eat almost anything, from floating organic matter to copepods to fish larvae. Probably because they sting and are overwhelmingly made of water, only a few types of fish will bother to eat them. They are preyed on by sea turtles, but there are far fewer of those than there used to be.

Finally, moon jellies have one of the most complicated and adaptable systems of reproduction in the animal world. They have several ways of reproducing asexually, and a couple of pathways of sexual reproduction. This gives them enormous potential to increase population size rapidly and with great flexibility based on local conditions. Give these guys an opening and they will take advantage quickly. That’s why they are found all over the world and that’s why we are seeing more and more of them.

As interesting and beautiful as they are, the increase in moon jellies around the world is creating problems. They do sting and are therefore a nuisance to swimmers. When there are lots of them, they can clog water intake pipes, and gum up and weigh down fishing nets. And, as mentioned above, they may be delaying the recovery of overfished resources.

For Your Viewing Pleasure:

This page shows the stages from polyp to floating medusa for Aurelia. Click on next. The first five slides are moon jellies.

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